1. Field of the Invention
This invention relates to blood pulse monitors and in particular to monitors which sense changes of blood volume through light sources and sensors in contact with the surface of the skin.
2. Description of the Prior Art
The need for proper and adequate exercise for both healthy persons and those with cardiac afflications is becoming more widely understood. With this new knowledge cardiologists, internists, general practitioners, and other physicians are prescribing exercise for the healthy as well as for those with heart problems. In addition, the general public is responding by participating in individual exercise programs such as tennis, biking, jogging, etc. This represents a significant trend away from traditional exercise routines, where they were previously practiced, for the most part, on a dedicated amateur or professional basis. Thus, the great majority of exercise was formerly handled on a planned and supervised basis which provided for control of the level of exercise. Even with that control, exercise physiologists are now finding that better control of amateur and professional exercise can dramatically improve results in a shorter period of time. Thus, control methods are starting to be used for organized exercise programs. This control can be achieved through normal physiological internal indicators such as fatigue or through external instrumentation monitoring pulse rate. Fatigue indications are usually adequate to insure that the exerciser does not injure himself, however, this does not insure proper amounts or levels of exercise, which is the main purpose of the program. The reasons for this are that the person may over-exercise during the beginning of the session and thus, not sustain his efforts long enough to be effective; the person may under-exercise but the fatigue symptoms resulting from the exercise may be so uncomfortable that he is discouraged from continuing further. Lastly, the level of effort to achieve acceptable exercise tends to change with factors such as general physical conditioning, fatigue level, ambient temperature and humidity, and emotional state. Monitoring pulse rate has been found to be an effective way of monitoring the adequacy of an exercise program.
Most instruments available on the market today for monitoring pulse rate, detect the heart beat by monitoring electrical signals emanating from the heart (electrocardiogram). In practice, it has been found that there are several serious drawbacks affecting the utility of these devices. These include the necessity for a good electrical contact between the body and instrument in order to reliably monitor electrocardiographic signals on a continuing basis. Furthermore, electrocardiographic techniques which assure good electrical contact require body electrodes which have to be changed periodically. These electrodes can also cause severe skin irritations. Ambulatory electrocardiographic techniques, while possible, are difficult with obese people and females with pendulous breasts. A single electrode configuration, as used for exercise monitoring, will not work on all people because of differences in the orientation of the electrical axis of the heart, especially in people who have suffered a heart attack. Lastly, the use of electrocardiographic techniques in colder climates can be inconvenient because of clothing problems. Because of the shortcomings related to electrocardiographic techniques, new techniques for monitoring pulse rate are required.
Recently, it has been found that pulse rate monitoring using light sensing techniques offers a simple and accurate approach to the problem. However, commercial devices based on this method have not been reliable because of problems associated with limitations in sensor technology, particularly those caused by ambient light, motion artifacts and, most importantly, inconsistent sensor behavior. Consequently, it has been observed that these units work inconsistently on some people and not at all on others. Some of these problems are overcome when infrared rather than visible light is used. The skin is somewhat transparent to infrared and so blood volume changes related to pulse rate can be monitored more easily. However, the basic problems with such devices still remain. The present invention is designed to correct the problems and provide a more reliable, lower cost instrument for the dynamic measurement of the pulse by a user or a medical professional.